To put it simply: The higher you fly, the more economical it is. On a long-haul flight, you can’t always “make” the desired altitude of 13 000 meters right away because the aircraft is too heavy due to all the kerosene it’s carrying. Only after some hours, when – depending on the type of aircraft – 20 000 to 30 000 liters are used up, can the pilots take the airplane “up a level” to the altitude where it consumes less fuel.

Are pilots authorized to determine their own cruising altitude?

Both the air-traffic controllers and the crew are in possession of the flight plan and route coordinates, times and, of course, the planned cruising altitudes, which are calculated before the flight. Pilots update these parameters during the flight, which means they also decide when to start flying at which cruising altitudes. These are then approved by the air-traffic controller – as long as no other aircraft nearby are flying at the same altitude. The rule is first come, first served.

Is there any connection between the vapor trails we see from the ground and an airplane’s altitude?

There’s a direct link between vapor trails and air temperature. Temperatures of about minus 40 degrees Celsius are required for a vapor trail to form. The higher the altitude, the lower the temperature. An aircraft has to fly at an altitude of roughly 8000 meters or higher in order for the air to be cold enough to produce a vapor trail.